The present invention relates to a method of manufacturing blown plastic articles, and more specifically, such articles having internal ribs to reinforce the structure thereof. Traditionally, blown plastic articles are processed in a manner which attempts to optimize strength and integrity of the container by various means of arranging the polymer molecules.
For example, an amorphous thermoplastic parison made of the crystallizable thermoplastics such as polyesters may be mechanically stressed at temperatures between the glass transition temperature and the melting point of the material, and then rapidly cooled, to align the molecular structure and achieve high strength in the direction of molecular orientation. However, if the parison is exposed to such temperatures for a sufficient length of time, crystallization occurs which results in a harder, stiffer material but sacrifices elasticity and clarity. While such sacrifices may be permitted for some containers, most commercial uses require some elasticity to accommodate stresses without brittle fracture, and require clarity for marketing reasons. Therefore, many processes attempt to optimize molecular orientation and minimize crystallization.
In the crystallizable thermoplastics, the rate of crystallization increases rapidly with temperature. Thus, orientation processes are generally limited to stressing the thermoplastic within a narrow temperature range above the glass transition temperature, called the orientation temperature range. Above that range and below the melting point, crystallization occurs rapidly and thus, it is commonly referred to as the crystallization temperature range. As an example, the orientation temperature range of polyethyleneterepthalate (PET), a preferred material for blown plastic bottles, is from 80.degree. C., the glass transition temperature, to 120.degree. C. The crystallization temperature range for PET extends from 120.degree. C. to the actual melting point which generally varies from 220.degree. C. to 260.degree. C. depending upon the degree of crystalline perfection and other variables.
It has been found, however, that oftentimes the highly-oriented, low crystallinity plastic bottles have insufficient structural integrity. Novel methods of achieving an improved balance between rigidity and elasticity are needed, especially in the plastic carbonated beverage bottle industry where the bottles need to be lightweight in order to economically compete with cans and glass bottles, and yet contend with such stresses as internal pressure, stacking loads, compressive stresses caused by capping machines in filling lines, etc.
One earlier approach to the deformation problem of lightweight and/or pressurized plastic containers had been to form reinforcing ribs along or around the external surfaces thereof. However, in order to provide adequate structural aid, the ribs preferably coextended with the surfaces of the container in the direction of the expected load. For example, if heavy vertical loads were anticipated, such as those incurred frequently during warehouse stacking, then the ribs were preferably coextensive with the height of the containers. One primary load of concern with plastic carbonated beverage bottles is the circumferential stress created by the internal pressure and, therefore, such bottles preferably include circumferential support ribs. The above-described rib arrangements, if external, then interfered with the labeling or decorative spaces on the outside of the container. In addition, the prominence of the display of external ribs required them to be designed in an aesthetic manner, thereby oftentimes resulting in waste of material and added weight. The present invention overcomes those disadvantages by providing a method of forming plastic containers having internal reinforcing ribs.
It is known in the prior art to extrude a tubular plastic parison having external reinforcing ribs which are reversed during the blow molding process to provide internal reinforcement to a blown container, as disclosed in U.S. Pat. No. 3,114,932 to Donnelly. The primary drawback of such a method, however, is the use of a special variable extrusion die which is costly because of the required machining and tooling of the die parts.
It is also known in the prior art to pre-blow a parison into a pre-blow mold having concave grooves patterned within its cavity walls, thereby forming an intermediate article with external bulbous ribs, and then final blowing the intermediate article within a blow mold such that the bulbous ribs are inverted to form internal ribs, as disclosed in commonly assigned U.S. Pat. No. 3,956,441 to Uhlig. The primary drawback of this method is that it requires the tooling and machining of a costly pre-blow mold, and decreases the efficiency of the entire blow-molding operation by including a separate pre-blow step.